FIG. 1 illustrates a GMSK transmitter. Such a transmitter may be used for mobile communications in Europe in accordance with Digital European Cordless Telecommunications (DECT) standard.
The GMSK transmitter 10 of FIG. 1 comprises a Gaussian Low Pass Filter (LPF) 12 and an FM modulator 14.
The transfer function of the Gaussian LPF is ##EQU1##
B.sub.t denotes the Gaussian Lowpass Filter (LPF) bandwidth which is illustratively 3 dB.
B is the cutoff frequency of the Gaussian filter and T is the data symbol time. Illustratively, BT=0.5
B.sub.tn is the equivalent noise bandwidth.
K is the transformation coefficient which relates B.sub.t to B.sub.m and is often considered to be one.
The Gaussian LPF filter 12 receives a Non-Return-to-Zero (NRZ) signal at its input. The output of the filter 12 is a baseband signal of frequency f.sub.b equal to 1.152 MHz. The baseband signal is inputted to the modulator 14. The FM modulator 14 is a minimum shift keying (GMSK) modulator. The output signal of the modulator 14 is a GMSK signal. The GMSK signal is in the intermediate band and has a frequency f.sub.IF =4f.sub.b.
A prior art detector 20 for a GMSK signal is illustrated in FIG. 2. The GMSK signal enters the intermediate band filter 22 with transfer function H.sub.r (t). The output signal of the filter 22 is EQU x(t)=cos[.omega..sub.IF t+.phi.(t)]
In the signal x(t), .omega..sub.IP =2.pi.f.sub.IF. The phase is .phi.(t). The filter 22 serves to band limit the signal x(t) and normalizes the envelope of x(t) to unity for all t. The phase .phi.(t) contains the information.
The detector 20 comprises the one unit delay 24, the 90.degree. phase shifter 26, and the multiplier 28. The multiplier 28 receives the signal x(t) directly, via path 29, and via path 30, which includes the delay 24 and phase shifter 26. The output of the multiplier 28 is connected to the zonal LPF 32. The output of the zonal LPF 32 is EQU y(t)=sin[.DELTA..phi.(T)]
where .DELTA..phi.(T)=.phi.(t)-.phi.(t-T). Thus .DELTA..phi.(T) represents the change in phase of the signal x(t) over a one symbol time interval. The function sin[.DELTA..phi.(T)] represents the sine of the change of phase of the signal x(t) over a one symbol time interval.
The output of the zonal LPF 32 goes to a decision circuit 34. The decision circuit outputs logic 1's and 0's according to the decision rule
______________________________________ sin[.DELTA..phi.(T)] &gt; 0 output logic 1 sin[.DELTA..phi.(T)] &lt; 0 output logic 0 ______________________________________
A transmitter and a detector for GMSK signals such as the transmitter 10 of FIG. 1 and the detector 20 of FIG. 2 is disclosed in Marvin K. Simon et al., "Differential Detection of Gaussian MSK in a Mobile Radio Environment" IEEE Transactions on Vehicular Technology, Vol. VT-33 No. November 1984, page 307.
One problem with the detector 20 of FIG. 2 is that an offset frequency will degrade the bit error rate performance. However, in the DECT standard an offset of .+-.50 kHz is acceptable.
If .omega..sub.IF =8.pi.f.sub.b +.DELTA..omega.=8.pi.f.sub.b +2.pi..DELTA.f, where .DELTA.f is the frequency offset, then the output of the zonal filter 32 is EQU y(t)=sin[2.pi..DELTA.f/f.sub.b +.DELTA..phi.(T)].
The signal y(t) is now the sine of the change in phase of the signal x(t) over a one symbol time interval plus a frequency offset term.
Thus, the signal used by the decision circuit 34 to decide between logic "1" and logic "0" is distorted due to the frequency offset. This distortion will degrade the bit error rate performance of the detector.
In view of the foregoing, it is an object of the present invention to provide a one bit differential detector for a GMSK signal which eliminates the distortion caused by frequency offset. More particularly, it is an object of the present invention to modify the detector circuit 20 of FIG. 2 to eliminate the dependence on the offset frequency of the output of the zonal filter 32.